Such a lamp is known from EP-A 0 562 679 which corresponds substantially to U.S. Pat. No. 5,387,837, commonly assigned herewith. The known lamp is of small internal diameter, for example 1.5 to 7 mm, and of a long length and, when filled with neon, for example, may be used for example as a signal lamp such as a tail lamp in automobiles, and has the advantage over an incandescent lamp that it emits its full light after 10 ms instead of 300 ms after being energized. A disadvantage of the known lamp is that its luminous flux and efficacy is comparatively low.
Presently, only low current and low wattage ND fluorescent lamps are available. These lamps have a relatively high cathode fall of about 180 volts and their light output is rather low (&lt;900 lm/m). Typically, axially configured, emitterless and hollow Fe--Cr--Ni electrodes (ferrules) are used in ND fluorescent lamps.
The high cathode fall (.about.180 volts) and high work function of axially configured, emitterless and hollow electrodes typically used in ND fluorescent lamps limit their use to lamp currents of less than 10 to 15 mA. Lower current results in a low light output (&lt;900 lm/m) and the high cathode fall reduces the lamp efficacy. High current ND fluorescent and neon lamps are highly desirable yet are non-existent. No electrodes are presently available for instant start ND fluorescent lamps with a current between 20 and 50 mA. The requirement for such lamps, among others, is a low cathode fall of, for example, less than 80 volts. There is therefore a need in the art for high current and high efficacy ND lamps. Such higher current ND fluorescent lamps may be used in automobile interior lighting or as backlights in laptop computers.
The cathode fall of an electrode in a lamp can be reduced by promoting electron emission. In traditional larger diameter and high current (&gt;200 mA) fluorescent lamps, a tungsten coil coated with triple carbonates (for example a mixture of barium, strontium and calcium carbonates) is used as the electrode. Consequently, these lamps have four terminals, two for each electrode on either side. During lamp manufacturing, in an extra process step, the carbonates are thermally converted into oxides in the lamp by passing a current through the tungsten coil. In the lamp, these oxides [(Ba,Sr,Ca)O] promote electron emission via thermionic emission when the electrode is heated to 1000-1300.degree. C., either by passing a heating current through the tungsten coil or by ion-bombardment. It would be desirable to have novel electrodes which do not require the extra thermal in-lamp processing step during manufacture, particularly since the step requires expensive processing time.
The electrodes presently used in instant start fluorescent and T2 lamps require a preheating current through the tungsten coil for optimum operation, thereby requiring a heating circuit in the ballast. It would be desirable to have novel electrodes which do not require a preheating current and in which the heating circuit could be eliminated from the ballast thereby lowering its cost. In these instant start lamps, electrode heating occurs during ignition due to ion-bombardment from the discharge. Therefore, the electrodes for instant start operation must withstand the sputtering. Such an electrode in which no in-lamp processing is required simplifies lamp manufacture and increases the lamp production rates.
An ND lamp requires single-lead electrodes because of geometrical constraints and therefore ion-bombardment is the only source of cathode heating. Due to the absence of a coil the use of carbonates in single-lead ND lamps would require external RF heating to convert them to oxides during manufacturing. This adds an additional, even more costly step to the manufacturing process. Therefore, new emitters, which do not need any in-lamp processing, are even more desirable for ND lamp electrodes.
Depending on the emitter used, a minimum temperature is necessary for electron emission. This temperature cannot be easily obtained in low-pressure discharge lamps, and especially ND lamps operating at low currents. Additionally, for ND lamp electrodes, the nature of the electron emission may be thermionic and/or secondary. Thermionic emission depends on the temperature and electric field. On the other hand, secondary emission depends on the ion current and temperature. Field emission is a third possibility for ND lamp electrodes and is dependent upon the strength of the electric field in front of the cathode. Ideally, the thermal conductivity of the electrodes should be low (&lt;20 watts/mK) such that the temperature of the glass feedthrough seal is sufficiently low. Low thermal conductivity will also allow the emitting surface to attain the thermionic emission temperature in the shortest possible time and therefore reduce lamp-blackening during starting. The electrical resistivity should be low to minimize resistive heat losses.